It is desired that an imaging lens installed in surveillance cameras, digital cameras, and other devices that use CCD, CMOS, or other photosensors be provided with the ability to faithfully reproduce the subject. CCDs themselves or CCD cameras have also been made smaller in recent years, which has been accompanied by an inevitable increase in the demand for miniaturization and compact design of the imaging lenses incorporated therein. CCD and other photosensors have also been provided with high resolution on the order of millions of pixels in contrast with CCD miniaturization. It has inevitably become necessary for the imaging lenses used in cameras having such sensors to also be capable of demonstrating high optical performance. In the past, in order to demonstrate high optical performance, aberration had been corrected using large numbers of lens elements.
A characteristic of a CCD, CMOS, or other photosensor is that the ray angle of each pixel incorporated therein is limited. In a camera equipped with an optical system that ignores this characteristic, the peripheral light intensity is reduced, and shading occurs. In order to compensate for these effects, methods have been employed whereby an electrical correction circuit is provided, or a microlens array that forms a pair with the photosensor is mounted or the like, and the angle at which light is received on the surface of the element is enlarged or the like. Alternatively, configurations have been adopted whereby the exit pupil is positioned as far away as possible from the image surface.
On the other hand, there must be a space between the imaging lens and the CCD in which a low-pass filter, infrared-blocking filter, or the like is inserted. A limitation therefore exists in that the back focus of the imaging lens must be lengthened to a certain degree.
An imaging lens having high resolution, a small number of lens elements, and a compact structure is disclosed in JP-A 2002-228922. The imaging lens disclosed therein is composed of four elements in three groups, and the second and third lens groups therein are composed of single lenses. An aspherical surface that contains an inflection point is also employed as the lens surface.